A physical downlink control channel (PDCCH) carries control information, such as downlink transmission allocation, uplink transmission grant, power control, paging, system information, and random access, etc.
A PDCCH consists of control channel elements (CCEs), and the number of the contained CCEs is defined as an aggregation level (AL). User equipment (UE) demodulates the PDCCH in a manner of blind detecting, and for each aggregation level, performs demodulation attempts on multiple possible PDCCH candidates.
A relationship between an aggregation level of a PDCCH and the number of CCEs and the number of PDCCH candidates needing to be detected is as shown in Table 1. All positions where the PDCCH possibly appears are referred to as search spaces.
TABLE 1Number of CCEs and number of candidates to which different aggregation levels of a PDCCH correspondAggregation levels Number of PDCCHof a PDCCHNumber of CCEscandidatesAL116AL226AL442AL882
FIG. 1 is a schematic diagram of a search space of a PDCCH. As shown in FIG. 1, 0˜(E−1) denote all CCE indices in an area of the PDCCH. For a certain aggregation level (in the following formulae, an aggregation level is denoted by “L”), a starting CCE index of any PDCCH is always an integral multiple of the L, and all PDCCH candidates are arranged consecutively.
In order to meet demands of LTE-advanced (LTE-A) for new scenarios, such as a heterogeneous network, coordinated multiple points, and carrier aggregation, etc., a control channel is also enhanced in LTE-A, which is referred to as an enhanced physical downlink control channel (ePDCCH).
An ePDCCH is transmitted in an area of a physical downlink shared channel (PDSCH), and is configured taking an ePDCCH set as a unit. For example, an ePDCCH set is currently defined as N physical resource block pairs (PRB pairs); where, N=2, 4, 8, and each physical resource block pair may be divided into 2 or 4 enhanced CCEs (eCCEs). Each ePDCCH set carries centralized ePDCCHs or carries distributed ePDCCHs. The centralized ePDCCHs occupy neighboring PRB pairs in a frequency domain, and their search spaces are defined in the ePDCCH set.
FIG. 2 is an example of a search space of centralized ePDCCHs when an aggregation level L=1. As shown in FIG. 2, an ePDCCH set where the search space is present contains N=4 PRB pairs, each PRB pair containing 4 eCCEs, and a number of ePDCCH candidates is 4. A principle of design of a search space of centralized ePDCCHs lies in distributing ePDCCH candidates in a range in the frequency domain as wide as possible, so as to obtain a frequency selectivity scheduling gain.
In a carrier aggregation (CA) scenario, a search space needs to provide support to cross-carrier scheduling; that is, control information on multiple component carriers (CCs) is transmitted in control channel resources of a current serving cell. For a search space of a PDCCH with an aggregation level L in Rel. 8, a CCE occupied by a PDCCH candidate with an index m may be obtained by using the formula below:
                              CCE          ⁡                      (                          m              ,              i                        )                          =                  L          ⁢                      {                                                            (                                                            Y                      k                                        +                    m                    +                                          M                      ·                                              n                        CI                                                                              )                                ⁢                mod                ⁢                                  ⌊                                                            N                      CCE                                        L                                    ⌋                                            +                              i                .                                                                        (        1        )            
In Formula (1), m denotes a PDCCH candidate index, i denotes an eCCE index in the candidate m, m=0, . . . , M−1, i=0, 1, . . . , L−1, M denotes a total number of candidates needing to be detected by UE in a blind manner, Yk is obtained by a Hash function and is related to a cell ratio network temporary identifier (C-RNTI) of the UE and a subframe number, NCCE denotes a total number of the CCEs contained in a PDCCH area, and nCI=0, . . . , NCC−1 is used to identify CCs that are scheduled, NCC denoting a total number of the CCs.
FIG. 3 shows a calculation result obtained by using Formula (1) when the number of CCs is 2 and L=1 (L=1 PDCCH cross-carrier scheduled). As shown in FIG. 3, without loss of generality, it is assumed here that Yk=0. It can be seen that PDCCH candidates of different CCs, i.e. CC0 and CC1, occupy CCE resources that are not overlapped each other.
For centralized ePDCCHs, in case of cross-carrier scheduling, an ideal principle of design of it should keep that ePDCCH candidates of different CCs are not overlapped each other, and at the same time, the ePDCCH candidates of each CC should be ensured to be distributed in a frequency range of a whole ePDCCH set as possible. On the basis of the single CC result in FIG. 2, a search space in cross-carrier scheduling may be obtained by using Formula (2) below (it turns back to a single CC scenario when nCI=0):
                              eCCE          ⁡                      (                          m              ,              i                        )                          =                  L          ⁢                      {                                                            (                                                            Y                      k                                        +                                          ⌊                                                                                                    m                            ·                                                          N                              eCCE                                                                                                            L                            ·                            M                                                                          +                                                                                                            n                              CI                                                        ·                                                          N                              eCCE                                                                                                            L                            ·                            M                            ·                                                          N                              CC                                                                                                                          ⌋                                                        )                                ⁢                mod                ⁢                                  ⌊                                                            N                      eCCE                                        L                                    ⌋                                            +                              i                .                                                                        (        2        )            
In Formula (2), meanings and values of m, i, L, Yk, M, nCI and NCC are as described above, which shall not be described herein any further, and NeCCE denotes a total number of the eCCEs contained in the ePDCCH set.
FIG. 4 is a schematic diagram of positions (ePDCCH cross-carrier scheduled) of ePDCCH candidates obtained by using Formula (2) when L=1. As shown in FIG. 4, ePDCCH-related configuration is: an ePDCCH set contains N=4 physical resource block pairs, each physical resource block pair containing 4 eCCEs, an aggregation level L=1, the number of the ePDCCH candidates is M=4, there are total 2 CCs, and Yk=0.
It should be noted that the above description of the background is merely provided for clear and complete explanation of the present disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of the present disclosure.